It is known that motors such as of the "universal" type or of the D.C. type are characterized by the ability of the rotor to rotate in one direction or in the other as a function of a choice in the mode of supplying the motor with electric current.
It is also known that the positioning of the brushes in contact with the commutator of the rotor for supplying the windings of the latter with electric current is important; indeed, it is known that it is preferable to place the brushes in line with an axial plane coinciding with the electric neutral of the field generated by the stator, and also that this plane is offset with respect to the axial plane corresponding to the mechanical neutral of the stator, respectively to one or other side thereof depending on the direction of rotation.
It has been proposed in U.S. Pat. Nos. 2,764,705 and 3,440,465 to make the pair of traditional brushes displaceable with respect to the stator between two limit positions corresponding to the respective positions of the electric neutral plane in opposite directions of rotation.
This solution makes it possible to optimize the positioning of the brushes as a function of the direction of rotation. However, it requires more complex electric circuitry for the brushes and the change in position of the brushes may be made only after they are electrically disconnected.
Further, the brushes wear differently depending upon the direction of rotation and upon whether the armature rotates more in one direction than the other. This also causes the brushes to operate under unfavorable mechanical conditions at the beginning of each phase of operation following a reversal in the direction of rotation.